This invention relates to processes for removing water from the ingredients and precursors for producing urethane polymers and prepolymers and, more particularly, to chemical processes for that purpose.
1. Field of the Invention
Polyurethane adhesives, sealants and the like are prepared by admixing one or more polyols with a polyisocyanate, a solvent, catalyst, and other additives such as thickening agents, thixotropic agents, colorants, etc. Such polyurethane products typically are packaged in a hermetically sealed container in partially cured form and cured upon exposure to air after being applied for the intended end use. The reaction of isocyanates with water is a known problem in a large number of processes for producing such polyurethane materials. The evolution of carbon dioxide from the isocyanate-water reaction and simultaneously cross-linking can cause foaming and formation of bubbles during and after processing, an excessive pressure build up in packed containers, an undesirable increase in viscosity and, in some cases, premature curing of the polyurethane.
One component, moisture-curing polyurethane resin compositions used as joint sealants, adhesives and coatings require special care in this regard. Thin layers of such compositions can be cured by the isocyanate-water reaction because the evolving carbon dioxide can escape harmlessly. However, when applied in thick layers or as a thixotropic material, latent curing agents, such as oxazolidines or imidazolidines, must be used to achieve a bubble-free cure. Also, such compositions normally are produced in sealed apparatuses under a blanket of a protective gas to prevent ingress of moisture and the starting materials, such as polyol, plasticizer, solvent, pigments, fillers and other additives, must be substantially anhydrous during production and storage to prevent partial cross-linking during storage.
2. Description of the Prior Art
One prior approach for removing moisture from the starting materials for polyurethane resin compositions has been to dry a mixture of polyol, solvents, pigments and filler by azeotropic distillation using a compound for entraining water, such as toluene and cyclohexane. This approach requires complex apparatus and high energy expenditures and, therefore, can be uneconomical for commercial production of some compositions.
Another prior approach is to admix with a mixture of the polyols, solvents, pigments and fillers, an inorganic compound capable of physically adsorbing or chemically reacting with water. Such inorganic compounds include alkaline earth metal oxides, such as magnesium oxide, calcium oxide and barium oxide and other oxidic compounds such as aluminum oxide, calcium sulfate or aluminum sulfate, perchlorates and other inorganic solids such as aluminum halides, magnesium halides and metal hydrides.
Such inorganic compounds are poorly soluble in the polyol-solvent mixture. Consequently, in order to obtain the surface contact between water and the inorganic compound required for the desired bonding, fresh surfaces of the inorganic compound must be continually generated by a shearing action with the attendant energy cost. This is difficult and sometimes impossible for thixotropic materials. Also, the drying time for this approach can be quite long (e.g., up to 24 hours or more), large amounts of such inorganic compounds are required, metal oxides can reduce the shelf life of isocyanate-terminated prepolymers and hydroxides produced by reaction with water can adversely affect certain physical and/or chemical properties of the cured polyurethane.
Organic compounds capable of chemically bonding with water such as ketals, particularly 2,2-dimethoxypropane (acetonedimethyl ketal), acetals, orthoformates, vinyl ethers, alkyl phosphites and tetraalkoxysilanes, have been used as drying agents. These compounds are readily soluble and provide good drying; however, their chemical reaction with water liberates alcohols, such as methanol or ethanol, which can react with isocyanate and act as chain terminator during synthesis of the prepolymer or polyurethane. Removal of alcohols usually is not economically feasible, so use of these organic compounds generally is impractical for many applications.
German Patent 1,245,590 discloses the use of low molecular weight isocyanates, such as p-toluenesulfonyl isocyanate, as drying agents for polyurethane compositions. These isocyanates can react with the hydroxyl groups of the polyols to be dried and become a chain terminator. For this reason, the pigment and filler are dried separately as a suspension in an inert solvent and the polyols are dried separately in another manner. In addition to the disadvantage of having to dry the pigment and filler in suspension and, consequently, the inability to prepare polyurethanes which are free of a solvent component, use of low molecular weight conventional isocyanates is unattractive because of their toxicity and lachrymatory properties and production of carbon dioxide.
Molecular sieves (zeolites) are currently widely used as water-binding agents for polyurethane ingredients. Thixotropic agents, such as hydrophilic pyrogenic silica, often are responsible for a large proportion of the water introduced by the polyurethane ingredients. Because of their insolubility, molecular sieves normally must be intensely admixed with a high speed mixer. Such mixing is undesirable for thixotropic compositions because of possible destruction of the thixotropy. To avoid this, a homogenized dispersion of molecular sieves in the polyols or solvent is prepared by roll milling before mixing with the other ingredients. Even with this additional step, the subsequent admixing does not always provide the intensive contact of the molecular sieves with the thixotropic agent required to produce the desired drying. Also, only about 25% of the pore volume of molecular sieves normally is available for water binding. Consequently, use of molecular sieves to remove large amounts of water is not economically practical in many applications.